1. Field of the Invention
This invention relates to a picture encoding method, a picture signal recording medium and a picture encoding apparatus. More particularly, it relates to a picture encoding method, a picture signal recording medium and a picture encoding apparatus employed in a system for storage-encoding and recording video signals of a moving picture on a picture signal recording medium, such as an optical disc, magnetic disc or a magnetic tape or in a system for transmitting video signals of a moving picture over a transmission path.
2. Description of the Related Art
For digitally recording picture data on a magneto-optical disc or a magnetic tape, or transmitting the picture data over a pre-set transmission medium, data is encoded or compressed for reducing the data volume.
The following description is made in connection with the use of the Moving Picture Image Coding Experts Group (MPEG) encoding standard as an encoding standard.
In encoding by the MPEG system, a video sequence is divided n terms of a group-of-pictures (GOP), made up of, for example, 15 frames, as a unit. Each frame is divided by the prediction system into the following three types:
an I-picture or an intra-frame coded picture; PA1 a P-picture or a predictive-coded picture which is a forward predictive-coded picture obtained on motion prediction from past and future P- or I-pictures; and PA1 a B-picture or a bidirectionally predictive-coded picture, obtained on motion prediction from past and future P- or I-pictures.
In such MPEG compression system, each frame picture is predicted with the prediction mode for the I-picture, P-picture or the B-picture, and the resulting prediction error is encoded and transmitted. Since basically the prediction error only is transmitted, the data volume can be compressed as compared to a case of directly transmitting picture data of each frame.
The relation between these picture types in shown in FIGS. 1A and 1B, of which FIG. 1A shows the relation of reference between an I-picture and P-pictures. In FIG. 1A, an I-picture is shown by cross-hatching, while P-pictures are shown by hatching. FIG. 1B shows the relation of reference between the B-pictures and the I-picture/P-pictures. By the pre-set number of pictures, as shown, there is formed a set of pictures or a picture string, termed a group-of-picture.
FIG. 2 shows a structure of each picture. Each picture or frame is made up of plural units termed slices resulting from slicing in a horizontal direction. Each slice, in turn, is formed by a pre-set number of macro-bocks, with each macro-block being made up of four luminance blocks composed of 8.times.8 pixels of luminance data and each one chroma block composed of 8.times.8 pixels of chroma data.
Meanwhile, in a chargeable TV system, scrambling is used for limiting viewing by other than contractors. For this scrambling, an electronic key is accorded to a receiver of each contractor. The picture data of broadcasting signals are modified, such as by inversion of white and black colors, in the blanking periods, such that scrambling cannot be resolved in the absence of the electronic key.
Similar scrambling is also demanded of a recording medium, such as a disc. That is, since a digitally recorded video disc is not deteriorated in picture quality on repeated copying, there is a risk that unauthorized copies be produced and offered in large quantities.
Thus it has been envisaged to use a publicly open key system enabling reproduction of only an authorized disc. The publicly open key system is not explained herein in detail since it belong to a prior art disclosed in, for example, U.S. Pat. No. 4,200,770 and other numerous publications.
Such scrambling cannot be applied satisfactorily in case of using the MPEG system. For example, the conventional method of applying an artifice in the blanking period cannot be used since the blanking periods are not used from the outset in the MPEG system.